Industrial products such as metallic materials are often inspected using ultrasonic waves to make sure that there are no harmful defects in them. In recent years, due to reduction in thickness of metallic materials for reduction in weight, changes of manufacturing processes for environmental measures, improvement in internal quality for longer life, and so forth, it has been necessary to detect ultra-minute internal defects of about 20 μm in diameter throughout the length and cross section of a metallic material. To inspect all of the produced metallic material products throughout their lengths and cross sections, it is necessary to inspect products being  carried in a production line. The carrying speed of products that require detection of ultra-minute defects is up to about 1000 mm/s. Therefore, it is necessary to detect ultra-minute defects of about 20 μm in diameter throughout the length and cross-section of a product being carried at a high speed of 1000 mm/s. In the case of inspecting a motionless product by scanning an ultra-sonic transducer, it is necessary to scan the ultrasonic transducer at a high speed of 1000 mm/s.
The above-described ultrasonic inspection apparatuses are called ultrasonic flaw detectors. In detecting the above-described internal defects by use of these apparatuses, techniques in which ultrasonic beams are electronically scanned are used for high-speed inspection purpose. One of these techniques, a commonly-used scanning technique called linear electronic scanning, will be described with reference to FIG. 13.
FIG. 13 is a block diagram showing a conventional ultrasonic inspections apparatus. In FIG. 13, reference numeral 101 denotes a transducer array. The transducer array 101 has, at the distal end thereof, many ultrasonic transducers (hereinafter simply referred to as “elements”) arrayed at regular intervals. Some of these elements are driven as a group so as to focus an ultrasonic beam on a preset position. In the shown example, the total number of elements is 64 (1011 to 10164), and the number of elements used as a group is eight. The elements are assigned element numbers 1 to 64. Reference letters B1 to B57 denote ultrasonic beams formed using the elements 1011 to 10164. Reference numeral 102 denotes a control circuit that controls transmitting; and receiving of the ultrasonic beams B1 to B57.
An outline of transmitting and receiving the ultrasonic beams B1 to B57 will be described. First, eight elements 1011 to 1018 are driven as a group, thereby transmitting and receiving an ultrasonic beam B1 that has a focal point (also called focus) on the center line of the elements 1011 to 1018. Next, elements 1012 to 1019 are driven as a group, thereby transmitting  and receiving an ultrasonic beam B2 that has a focal point on the center line of the elements 1012 to 1019. Similarly, elements to be driven are shifted by one element. Finally, elements 10157 to 10164 are driven, thereby transmitting and receiving an ultrasonic beam B57. In such operation, a test object is electronically scanned with an ultrasonic beam at intervals equal to the element arrangement interval. The control necessary for transmitting and receiving of the above-described focused ultrasonic beams and for electronic scanning is performed by use of the control circuit 102 connected to the transducer array 101.
Focusing of transmitted ultrasonic beams is possible by changing the timing of an electric pulse applied to each element to transmit ultrasonic waves, in the group of elements. Focusing of receiving beams can be achieved by delaying signals received by the group of elements, element by element, by a specific time, and adding them.
It is said that the above-described linear electronic scanning is about 20 times faster than mechanical scanning of an ultrasonic single probe. However, in the case of inspecting a test object carried at a high speed of about 1000 mm/s in a transfer line, for example, of metallic materials using the above-described linear electronic scanning, a significant portion of the test object passes before electronic scanning is completed. Therefore, oversights can occur in inspection.
Publications discussing speeding up the inspection using linear electronic scanning include Japanese Unexamined Patent Application Publication No. 3-248058. Japanese Unexamined Patent Application Publication No. 3-248058 proposes speeding up linear electronic scanning by “an ultrasonic inspection apparatus that scans ultrasonic beams along an array of many ultrasonic transducers, the apparatus including a beam region dividing means that divides all of the ultrasonic beams into a series of beam regions, a beam region selecting means that  selects the beam regions in a predetermined order, and a shifting means that sequentially shifts an ultrasonic beam in a selected beam region every time the beam region is selected.”
Publications that discuss speeding up the cross-sectional inspection of a test object include Japanese Unexamined Patent Application Publication No. 2003-28846. Japanese Unexamined Patent Application Publication No. 2003-28846 proposes speeding up the cross-sectional inspection of a test object by “an ultrasonic flaw detector including an ultrasonic transducer array having a plurality of transducers that can be arranged along the surface of a test object, an exciting means that excites each transducer of the ultrasonic transducer array with a spike pulse, a waveform memory that stores ultrasonic echoes received by each transducer as waveform data of each transducer, a phase summing means that reads the contents of the waveform memory in which waveform data of each transducer are stored and that makes phase summing of them using a summer, and a focusing means that gives, during reading of the waveform memory, the waveform memory address corresponding to the beam path distance of the dynamic focus set to an arbitrary position within the electronic scanning range.”
However, in Japanese Unexamined Patent Application Publication No. 3-248058, the fact remains that scanning of ultrasonic beams is performed by electronic switching, and it is far from a solution to the above-described problem of the oversight in the inspection.
In Japanese Unexamined Patent Application Publication No. 2003-28846, in forming the focuses of the receiving beam by use of all of the received signal data of the transducer array stored in the waveform memory, it is necessary to sequentially shift the focal depth. Therefore, this process disadvantageously takes time. In paragraph [0042] of Japanese Unexamined Patent Application Publication No. 2003-28846 is shown an example in which a cross-sectional inspection ends in 1 ms. However, in the case where the speed of a test object is, for example,  1000 mm/s (60 mpm), the inspection of the test object can only be performed at intervals of 1 mm. If there is a circular planar defect about 100 μm in diameter in the test object, the probability that an ultrasonic beam hits this defect at a right angle is smaller than 1/10.
In Japanese Unexamined Patent Application Publication No. 2003-28846, a focus of a receiving beam is formed at a specific position by phase summing all of the n received signals received by n elements. 200 is taken as an example of n. Since the diameter of a receiving beam at the focal position is inversely proportional to the size of the aperture, a large n might seem to be preferable from the viewpoint of the defect detectability and the resolution. However, each individual ultrasonic transducer (also called “element”) constituting the transducer array has a certain width in the array direction, the receiving beam directivity of each individual ultrasonic transducer is limited to a narrow angular range. Assume that, for example, the nominal frequency of the transducer array is 5 MHz, and the element width in the array direction is 0.8 mm (this is a typical element width of a common 5 MHz transducer array). In this case, the angle at which the receive efficiency is within −6 dB in comparison to the receive efficiency on the central receive beam axis (called receive directivity) is about 12° (with respect to the central beam axis). Assume that, using this transducer array and using only elements whose receive directivities with respect to the focus are within −6 dB, a focus is formed at 50 mm from the transducer array. The element located just above the focus is denoted as element i. An element j whose receive directivity with respect to the focus is within −6 dB is located about 11 mm from the element i. Since the element width is 0.8 mm, the element j is the 13th or 14th element from the element i. Therefore, in the above-described case, the total number of elements that mainly contribute to the focus of a receive beam is a little less than 30. As described above, when the technical idea shown in Japanese Unexamined Patent Application Publication No. 2003-28846 is applied to the  common case, the phase synthesis of more than 80 percent of the elements is wasted. In addition, in the case where the apparatus shown in Japanese Unexamined Patent Application Publication No. 2003-28846 is applied to the on-line flaw detection in a manufacturing premise, the factory-specific cyclic noise included in the signals received by more than 80 percent of the elements that hardly contribute to formation of a focus increases by addition, and therefore a noise signal of large amplitude tends to be generated. Since a noise signal of large amplitude causes false detection, it is the most detestable problem in the on-line flaw detection.
It could therefore be helpful to prevent inspection omissions when ultrasonic flaw detection using a transducer array is applied to inspect a test object being conveyed at a high speed or when a test object is inspected by moving a transducer array at a high speed. In addition, it could be helpful to provide methods and apparatus free from noise of large amplitude.